In a field of acoustics, processing of MFB (Motional Feed Back) is known from the past. In the processing of MFB, an electric signal obtained from motion of a speaker diaphragm is detected. The detected electric signal is positively fed back or negatively fed back to an audio signal, thereby controlling the motion of the diaphragm of a speaker unit. The MFB processing of positive feedback enables reproduction of the audio signal with a sense of reverberation. The MFB processing of negative feedback restrains unfavorable sound in low frequencies. Audio systems realizing MFB by analog devices are disclosed in JP-A-10-164685 (Patent Document 1), JP-A-10-070788 (Patent Document 2) and JP-A-2004-200934 (Patent Document 3).